Radiometric Measurement Traceability Paths for Photovoltaic Calibrations. Howard W. Yoon Physical Measurement Laboratory NIST

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1 Radiometric Measurement Traceability Paths for Photovoltaic Calibrations Howard W. Yoon Physical Measurement Laboratory NIST

2 Solar energy and PV Solar radiation: free and abundant! Photovoltaics (PV): direct energy conversion from solar to electricity PV is a clean renewable energy source (no air pollution, quiet operation etc) Significant reduction in costs (from $1000/W in 1959 to ~$1/W in 2012) Have reached grid parity in California, Texas, New York and others by Department of Energy Sean Shaheen, U of Denver

3 Stats PV at a glance PV tech roadmap and our mission Key issue for manufacturers: increase panel efficiency and power ratings, Need for distinguishing leading suppliers in a crowded market. Accurate power rating measurements and verification Fast and reliable Inline manufacturing tools Module degradation and monitoring in the field

4 Standard Test Conditions (STC) for power rating and efficiency STC: Air Mass 1.5 (AM1.5), 25 deg C, 1000 W/m^2

5 Spectral mismatch error Spectral mismatch parameter (M): Measure of error in PV device testing from 1. Mismatch between spectral response of PV test device and PV reference cell 2. Mismatch between the test light source and the reference spectrum (ie, AM 1.5) M λ2 λ4 E ( λ) R ( λ) dλ E ( λ) R ( λ) dλ s t r r λ1 λ3 λ4 λ2 = E ( λ) R ( λ) dλ E ( λ) R ( λ) dλ s r r t λ3 λ1 ASTM E973 Es Er ( λ) ( λ) : spectral irradiance of the source, W m -2 nm -1 Rr ( λ ) : spectral response of reference cell, A W -1 : reference spectral irradiance, W m -2 nm -1 Rt ( λ ) : spectral response of test cell, A W -1 To calculate this error: must establish procedure to determine the spectral irradiance of our indoor lighting source & spectral responsivity of both the reference cell and the device under test. For spectral irradiance, calibrated spectroradiometers directly traceable to NIST FEL lamps For spectral responsivity, two facilities have been established.

6 Spectral responsivity Solar cell spectral responsivity SR(λ): The measurement of the wavelength dependence of the photocurrent relative to the # of incident photons. SR(λ) = I ph (λ)/p(λ) Spectral responsivity of some PV technologies Essential for solar cell efficiency and rating certification under STC. SR varies among different cell technologies. SR of reference cells do not necessarily match SR of device under test.

7 Three different methods for calibrating solar cells 1. Outdoor method 2. Solar simulator method 3. Differential Spectral Responsivity (DSR) method

8 Si Trap Detector Outdoor Method (NREL) Cryogenic Electrical Substitution Radiometer NIST SSD Standards Si Trap Detector Filter Radiometer HTBB Spectral Irradiance Lamps Spectroradiometer World Radiometric Reference (WRR) 0.6 % (k=2) Cavity Radiometer, E TOT Outdoor Relative Solar Spectral Irradiance, E R,OUT Si Working Standards Relative Spectral Responsivity, s R,λ AM1.5, E AM1.5G Solar Cells, Modules I(OUT) I STC W 1000 m E 2 ( AM1.5G) = I( OUT ) MMF TOT World Photovoltaic Scale (WPVS) 1.7 % (k=2) MMF = s R,λ E E AM1.5G AM1.5G s E R,λ R, OUT E R, OUT

9 How is the broad-band total irradiance scale (World Radiometric Reference (WRR)) disseminated? Every 5 years the International Pyrheliometer Comparisons are held. The Eleventh IPC (IPC-XI) took place at PMOD/WRC (Davos, Switzerland) from September 27 to October 15, Eighty-seven participants came from 40 countries to calibrate 99 pyrheliometers.

10 NIST has 3 Eppley AHF pyrheliometers Recommended by NREL who has provide the custom software to run this unit.

11 Calibration of Field Instruments Absolute Cavity Pyrheliometer Commercial Pyrheliometer Pyranometer

12 Si Trap Detector Solar Simulator Method Cryogenic Electrical Substitution Radiometer Si Trap Detector Filter Radiometer Si Working Standards HTBB Spectral Irradiance Lamps I STC ( AM1.5G) = I( SIM ) s R, λ s R, λ E AM1.5G E SIM Relative Spectral Responsivity, s R,λ Spectroradiometer Indoor Simulator Spectral Irradiance Absolute Spectral Irradiance, E SIM AM1.5, E AM1.5G Solar Cells, Modules I(SIM)

13 PV testing methodology Testing usually performed under indoor lighting conditions. Issues to consider: 1. Quality and rating of the solar simulator A. Spectral irradiance match to the Standard Test Conditions (STC)(AM 1.5) B. Uniformity of the illumination C. Temporal stability of the light source 2. Spectral responsivity and calibration of a reference cell. 3. Spectral responsivity of the test cell or module. 4. Determination of a spectral mismatch factor, M. 5. Current-voltage (I-V) measurements of the device. 6. Correction of the I-V curve using M. 7. Determination of the module parameters, including power conversion efficiency and power rating.

14 PV Indoor testing method Cell & Module DAS Programmable Delay Solar Cell Solar Module Si InGaAs Power Supply Place specimen and calibrated reference cell under a simulator Adjust simulator output until reference cell produces an output close to its certified value at standard conditions (temp, spectrum) Determine output of specimen by performing current-voltage measurements. Make slight corrections based on reference cell to normalize the specimen's output to standard conditions.

15 NIST Solar simulator 1. IEC Standard Class AAA Simulator 1. Spectral match: for 6 wavelength-intervals 2. Irradiance non-uniformity: ±2 % 3. Irradiance temporal instability: ±2 % 2. Flash Plateau: 36 ms 1. IV scan interval: 1 ms 2. Variable scan delay : 1 12 ms 3. Irradiance Range: ~ W/m 2 4. Maximum irradiated area: 2.4 m diagonal 5. Spectral composition filter

16 Spatial uniformity measured with solar cell X-distance [ cm ] Y-distance [ cm ] Noramalized Irradiance [ W / m 2 ]

17 Calibration for spectral irradiance responsivity Calibrations using the NIST 1000 W FEL lamps at 50 cm (integration time of 200 ms)

18 Temporal control of the measurement Reference Cell Output (Suns) Pulse duration of 36 ms Simulator Flash Profile Measurement duration of < 10 ms Percent difference from mean [ % ] Elapsed Time Since 10% Irradiance (ms) Controlled delay

19 Si and InGaAs PDA nonlinearity corrections NMOS Si PDA InGaAs PDA Percent correction, (t max = 600 ms ) R T Percent correction, (t max =160 ms ) R T Integration Time [ ms ] Integration Time [ ms ] Measurements performed outside R C 1 S = RT T S τ D

20 Outdoor measurements compared to ASTM G UV-VIS NIR 1400 Spectral Irradiance (W/cm^3) Wavelength (nm)

21 Comparison of indoor simulator with ASTM G Solar Simulator ASTM G173 Spectral Irradiance W/(m^2 nm) Wavelength [ nm ]

22 Classification according to IEC ratios Beginning Wavelength [ nm ] Ending Wavelength [ nm ] Integrated Irradiance [ W/m^2] Measured Percentages IEC Standard Percentages IEC Ratio Class Rating % 18.4% 0.98 A % 19.9% 0.82 A % 18.4% 0.80 A % 14.9% 0.97 A % 12.5% 1.21 A % 15.9% 1.33 B Total

23 Uncertainties of the NIST solar simulator Uncertainty Component Si [ % ] InGaAs [ % ] Wavelength Range 300 nm to 950 nm 950 nm to 1700 nm 1 NIST Spectral Irradiance Scale Signal Noise Temporal Stability of Spectroradiometer Spectroradiometer Stray Light Linearity Correction Wavelength Calibration Total Uncertainty Expanded Uncertainties (k=2)

24 Differential Absolute Responsivity (DSR) NIST Method Si Trap Detector Cryogenic Electrical Substitution Radiometer Si Trap Detector Filter Radiometer HTBB Spectral Irradiance Lamps Spectroradiometer ( AM.5G) s I STC 1 = A, λ EAM1. 5G Relative Spectral Irradiance Si Working Standards Absolute Spectral Responsivity, s A,λ 1.0 % (k=2) AM1.5, E AM1.5G Solar Cells, Modules World Photovoltaic Scale (WPVS) 1.7 % (k=2)

25 Conclusion 1. Utilization of photovoltaic energy generation is increasing rapidly, and electrical power generated through PV has now achieved grid parity in many states in the US. 2. Radiometric measurements and standards are critical in assigning conversion efficiencies and power ratings.